How do I secretly buy my girlfriend a surprise gift when we share all our bank accounts?

So my girlfriend’s birthday is coming up in a few weeks and I want to get her something really nice as a surprise. The problem is we’ve been living together for 2 years and have everything linked to our shared checking account like all our cards, online shopping, everything goes through the same account that we both monitor pretty regularly.
I found the perfect gift for her (it’s this necklace she mentioned months ago that she thought was beautiful but too expensive) but I have no idea how to buy it without her seeing the charge immediately. She checks our account like every other day to keep track of our budget. I thought about asking my brother to buy it and paying him back but idk if I should do it or not. I could try to time it right after a big expense like rent when there’s a lot of activity, but her attention to detail is insane like she would notice it right away. We don’t really keep separate money anymore since we combined everything when we moved in together. I know some couples keep separate ‘fun money’ accounts but we never set that up. Any ideas for how to pull this off? Thanks!